This proposal is designed to seek basic information on the properties of unprimed L3T4+ and Lyt-2+ T cell subsets responding to H-2 alloantigens. Particular emphasis is placed on studying Lyt-2+ cells since less is known about these cells than L3T4+ cells. Three broad areas of investigation are proposed: 1) Variation in the responsiveness of Lyt-2+ cells. Although purified Lyt-2+ cells can give very high responses to class I (but not class II) H-2 alloantigens in the absence of added lymphokines, certain class I differences are much less immunogenic than others. Moreover, the general level of responsiveness of Lyt-2+ cells can vary considerably from one strain to another. Various approaches, including the use of thymus-grafted chimeras and recombinant inbred mice, will be used to seek further information on these two levels of variability in the responsiveness of Lyt-2+ cells. 2) Stimulatory requirements for Lyt-2+ cells in vitro. Whereas L3T4+ cells respond solely to Ia+ cells, Lyt-2+ cells can be stimulated by class I differences expressed or certain Ia- cells. Whereas normal allogeneic T cells are nonstimulatory for Lyt-2+ cells unless supplemented with lymphokines, Lyt-2+ cells respond strongly to a subpopulation of Thy 1- Ia- cells present in spleen and marrow. Unprimed Lyt-2+ cells also respond to certain Ia- tumor cells, including T tumors. By scrutinizing the response of Lyt-2+ cells to these tumor cells, we hope to obtain information on what signals are needed for Lyt-2+ cell induction. 3) In vivo functions of T cell subsets. Four approaches will be used to seek further information on the behavior of T cell subsets in vivo. First, the Winn assay will be used to examine the capacity of Lyt-2+ cells to reject various types of allogeneic tumor cells; preliminary work has shown that mixing allogeneic tumor cells with unprimed Lyt-2+ cells and injecting the cells subcutaneously into irradiated mice prevents the appearance of macroscopic tumors. Second, studies with bone marrow chimeras will be used to determine whether Lyt-2+ cells have the capacity to recognize allo class I molecules on non-marrow derived cells. Third, detailed information will be sought on the capacity of L3T4+ and Lyt-2+ cells to cause lethal graft-versus-host disease directed to allo class I vs. class II differences. Fourth, further information will be sought on the capacity of Lyt-2+ cells to mediate rejection of skin allografts.